A new study suggests that acts of kindness, generosity and cooperation are highly contagious. And it takes only a handful of individuals to really make a
difference.

ScienceDaily has the details of the findings, but here's a diagram and an explanation that may help elucidate this "pay it forward" effect of kindness. Hopefully it doesn't make you dizzy like it made me:

This diagram illustrates how a single act of kindness can spread
between individuals and across time. Cooperative behavior spreads three
degrees of separation: if Eleni increases her contribution to the
public good, it benefits Lucas (one degree of separation), who gives
more when paired with Erika (two degrees of separation) in period 2,
who gives more when paired with Jay (three degrees of separation) in
period 3, who gives more when paired with Brecken in period 4. The
effects also persist over time, so that Lucas gives more when paired
with Erika (period 2) and also when paired with Lysander (period 3),
Bemy (period 4), Sebastian (period 5), and Nicholas (period 6). The
effect also persists at two degrees of separation, as Erika not only
gives more when paired with Jay (period 3), but also when paired with
Harla (period 4) and James (period 5). All the paths in this
illustrative cascade are supported by results in the experiments, and
it is important to note that if Eleni decreases her initial
contribution then her uncooperative behavior can spread and persist as
well. (Credit: Courtesy James Fowler, UC San Diego)